Quantum-dot technique builds 'better light bulb'

Portland, Ore. - Vanderbilt University researchers claim their quantum-dot approach to the generation of tunable broad-spectrum white light simplifies solid-state lighting. The quantum-dot light bulb, invented by professor Sandra Rosenthal at the Nashville, Tenn., institution, uses a single size of nanocrystal to produce white light when irradiated with commercially available blue LEDs.

The work shows that building a "better light bulb" does not require pumped lasers, exotically formulated phosphors or integrated quantum wells and nanocrystals, Rosenthal said: All you need to do is coat blue LEDs with her broad-spectrum quantum dots.

"What is unique about our approach is that it is a single size of nanocrystal, or quantum dot, with a single surface treatment that gives white light," she said. "Other researchers modify the nanocrystal surface or size to get different colors and then make a mixture to get white light." Rosenthal performed the research with Vanderbilt doctoral candidate Michael Bowers and with postdoctoral assistant James McBride.

The quantum-dot light bulbs are predicted to last as long as their LEDs-up to 50,000 hours, or 50 times as long as a normal light bulb. According to the Department of Energy, LED lighting could reduce U.S. energy consumption for lighting by 29 percent by 2025, for a $125 million saving.

When excited by a specific wavelength, quantum dots emit light that is shifted to a different wavelength. Usually the emitted light is at a single wavelength and is determined by the nanocrystal size-for example, a 10-nanometer diameter for red-but surface treatments can be used to create a cold bluish white light. Rosenthal's group, however, discovered a size and surface treatment combination that enables a single quantum dot to emit a full spectrum combination of light, resulting in warm yellowish white emission.

High-temp reactionIn the pyrolisis reaction that fabricates the dots, "molecules containing cadmium and selenium are injected into a very hot solution of trioctylphosphine oxide," said Rosenthal. "At this very high temperature, the molecules degrade to yield free cadmium and selenium, which react to form nanocrystals. We then quickly stop the reaction to freeze the size, and a solvent passivates the surface of the nanocrystals so that they don't form into aggregates."

After repeated attempts at making smaller and smaller nanocrystals, the researchers found that the semiconductor they chose-the compound of cadmium and selenium-naturally aggregates into quantized ultrasmall aggregates of as few as 33 atoms. The researchers called that the "magic size." The resulting quantum dots-half the size of normal nanocrystals-not only emitted full-spectrum light but appeared to exhibit photonic surface emission.

"In larger nanocrystals, which produce light in narrow spectral bands, the light originates in the center of the crystal. But as the size of the crystal shrinks to the magic size, the light emission region appears to move to the surface of the crystal and broadens out into a full spectrum," said Rosenthal.

To coat the plastic surface of a blue LED, the researchers merely mixed their quantum dots into a transparent polymer and painted it onto a blue LED. Coating the LED takes less than an hour, compared with days or weeks for the surface treatments other labs require to achieve bluish white emission, Rosenthal said.

The researchers plan to optimize their process to improve their quantum-dot yields and lower the cost of producing them. They also plan to devise tests to verify their hypothesis that photonic surface-emission is the mechanism by which the quantum dots are able to glow with full-spectrum light.

The researchers hope to "demonstrate electroluminescence from the white-light nanocrystals," said Rosenthal. That would open the door to white-light emission directly from the input of electricity.

Other groups have reported electrically exciting quantum dots to the point of electroluminescence, but those approaches all produced colored light. Rosenthal hopes that stimulating her photonic surface-emitting quantum dots electrically will yield an all-semiconductor white light bulb that does not rely on exotic compounds. The quantum dots theoretically could be sprayed on any surface to turn it into a light bulb producing a variable rainbow of shades.